1. Field of the Invention
This invention relates to apparatuses for the formation of images such as electrophotographic copying machines, printers, and the like.
2. Description of the prior art
With a conventional apparatus for the formation of images such as an electrophotographic copying machine, an electrostatic latent image formed on a photosensitive drum is developed by a toner, resulting in a toner image. The toner image is then transferred onto a sheet of copy paper by a transfer unit with a corona charger, and the copy sheet with the toner image is further separated from the photosensitive drum by a separating unit with a corona charger, so as to be conveyed to a fixing unit by a conveying unit. The fixing unit fixes the toner image onto the copy sheet.
For the conveying unit to convey the copy sheet with the toner image to the fixing unit, a set of conveyer belts which move in circulation are usually used. Normally, the set of conveyer belts and the separating unit are located so that a space is formed therebetween. When the conveyer belts are operated, ambient air around the belts flows with the circulation of the belts, so that the air may flow through the space toward the photosensitive drum. Moreover, since the conveyer belts are spaced at given spacings in the direction at right angles to the direction of conveyance, there is a possibility that the air may also flow through the spacings toward the photosensitive drum. Furthermore, a shield case of a corona charger serving as a transfer unit or a separating unit is usually formed with an opening for discharging ozone therethrough, so that the air may flow through the opening toward the photosensitive drum.
In the fixing unit which is adjacent to the set of circulating conveyer belts, the toner on the copy sheet is melted by the application of heat and fixed onto the copy sheet with the application of pressure. Because the copy sheet is heated in the fixing unit, water contained in the copy sheet evaporates into ambient air. The hot ambient air containing vaporized water around the fixing unit is moved with the circulation of the conveyer belts, so as to be blown onto the surface of the photosensitive drum through the space between the set of conveyer belts and the separating unit.
In the transfer unit and separating unit, both of which are corona chargers, the corona charge gives rise to a dielectric breakdown in the ambient air, so that the ambient air is ionized, resulting in the formation of ozone. The ozone is also blown onto the surface of the photosensitive drum with the circulation of the conveyer belts.
Thus, with the conventional electrophotographic copying machine, the hot air containing vaporized water and ozone flows upward through the space between the conveyer belts and separating unit and is blown onto the surface of the photosensitive drum.
When ozone is blown onto the surface of the photosensitive drum, there is a possibility that the photosensitivity of the drum is lowered. When the hot air containing vaporized water is blown onto the surface of the photosensitive drum, there is also a possibility that vaporized water, which is contained in the hot air, condenses on the surface of the photosensitive drum, so that a distinct toner image is not formed on the photosensitive drum.
Japanese Laid-Open Utility Model Publication No. 61-128714 discloses an electrophotographic copying machine, in which a heater is disposed below the photosensitive drum in order to prevent condensation on the surface of the photosensitive drum. With this copying machine, condensation of water on the photosensitive drum is prevented by applying heat to the photosensitive drum by means of a heater. However, the copying machine does not prevent the hot air containing water from being blown onto the surface of the photosensitive drum by the circulation of the conveyer belts, so that it is impossible to efficiently prevent condensation on the photosensitive drum. Moreover, it is also impossible to prevent ozone formed by the transfer unit and separating unit from being blown onto the photosensitive drum, so that the deterioration of photosensitivity of the drum cannot be prevented.
In order to overcome the above-mentioned problems, it is possible to propose an electrophotographic copying machine which is provided with an exhaust unit below the transfer unit and separating unit. The exhaust unit forcibly moves the air downward, so that damaging air is not blown onto the photosensitive drum. However, the copying machine provided with an exhaust unit becomes large, and the exhaust unit is not capable of efficiently preventing the flowing of air which occurs in the vicinity of the photosensitive drum with the circulation of the conveyer belts. Moreover, when the photosensitive drum is heated by a heater to prevent condensation on the photosensitive drum, the hot air is exhausted by the exhaust unit, resulting in the decreased efficiency in applying heat to the photosensitive drum.